In recent years there has been a growing interest in increased automation in the control of the drive train of motor vehicles, and most especially in control of the drive train of large trucks. The use of automatic transmissions in passenger automobiles and light trucks is well known. The typical automatic transmission in such a vehicle employs a fluid torque converter and a gear train with hydraulically actuated clutches and brakes for selecting the final drive ratio between the engine shaft and the drive wheels. This gear selection is based upon engine speed, vehicle speed and the like. Another type of transmission, which is for heavy trucks, employs an automatic friction clutch instead of a fluid torque converter. Such a transmission and its clutch control is further described in U.S. patent application Ser. No. 772,204, filed Oct. 7, 1991 and entitled "Closed Loop Launch and Creep Control for Automatic Clutch", and U.S. patent application Ser. No. 772,778, filed Oct. 7, 1991 and entitled "Closed Loop Launch and Creep Control for Automatic Clutch with Robust Algorithm", which are assigned to the assignee of this invention.
The usual technique for controlling the various brakes and clutches in the transmissions has been through a fluid actuator, usually hydraulic but which can be pneumatic, which is supplied from a source of fluid pressure through a solenoid valve operated by an electronic control. Such controls determine the rate of actuation to achieve smooth and efficient torque transfer during the transition between engaged and disengaged states. In particular, such controls use pulse width modulation to determine the rate of actuation; pulses are emitted at a fixed frequency and the pulse width is varied in proportion to the desired duty cycle. Rapid actuation is thus afforded by a large pulse width. A result of that mode of control is that each large pulse width results in a large increment of actuator movement so that fine resolution or smooth movement is not possible where a moderate or a large actuation rate is required. Further, slow actuation requires a minimum pulse width. If an initial minimum width is preset to be large enough to surely cause actuation, it will be larger than the minimum required width. Otherwise it is necessary to begin near zero pulse width and increment the pulse width at each scheduled pulse event resulting in a time delay until an effective pulse width is attained. Such a time delay, when applied to a closed loop control can cause instability.